Guides

Installation and safety

230 V can kill. Here's what applies before you screw a Shelly in behind a light switch or into the consumer unit — and when to leave it to an electrician.

Most Shelly devices connect to mains power — 230 V across most of Europe. This isn't a USB cable. Getting it wrong can kill you or start a fire. This chapter goes through what Shelly's own safety guides say, and where the line is for what you're allowed to do yourself.

Cut the power and verify it's dead

Shelly's own safety guides (verified against, among others, the Shelly Bypass User and Safety Guide on kb.shelly.cloud — the same wording recurs across the guides for most mains-powered devices) are unambiguous on this point:

• Turn off the breaker/fuse for the circuit before touching any wires.
• Use a suitable voltage tester to confirm the wires are actually dead before you continue.
• Every new change to the wiring must be made only after you've re-confirmed there's no voltage present at the device's terminals.
• Don't install the device where it can get wet.
• Don't use a device that's been damaged, and don't attempt to repair it yourself.

Flipping the wall switch to "off" is not enough on its own — it typically only breaks one of the conductors, and depending on how it's wired, other terminals may still be live. Cut power at the breaker, and measure.

When you must hire an electrician

Shelly's safety guides state, plainly, for essentially every mains-powered product: "Mounting/installation of the Device to the power grid has to be performed with caution, by a qualified electrician!" That wording is verified in several of Shelly's own guides (including the Bypass guide) and recurs as a general pattern.

Beyond Shelly's own recommendation, most countries have a legal requirement that doesn't come from Shelly: a fixed installation — meaning a device wired directly into a back box, junction box, or consumer unit, with conductors screwed into terminals — may only be carried out by someone with the required electrical qualification (a certified electrician). This applies to devices like Shelly 1, Shelly Pro 4PM and similar relay modules mounted behind a switch or in the consumer unit.

Devices connected via a plug into an ordinary wall socket, such as Shelly Plus Plug S, aren't normally considered a fixed installation and don't require an electrician to plug in — but the wiring inside the plug itself was already done by the manufacturer, which isn't the same as wiring conductors yourself.

This point about the law is not taken from Shelly's documentation — Shelly simply says "hire a qualified electrician" without explaining why or when it's a legal requirement. Rules on fixed wiring vary by country. If you're not sure whether your installation counts as a fixed installation, check your national electrical safety regulations or ask an electrician before you start.

Live, neutral and protective earth

L and N exist on practically every mains-powered Shelly device and supply the device's own electronics with power — that's not the same thing as the load circuit's I/O terminals, which switch whatever you've connected (the lamp, the motor). See Terminals and markings for what each letter means on your specific device, and Colour coding for how the housing and terminal colour reveal the model and generation.

The protective earth PE is not a neutral. Wiring an earth conductor as if it were neutral to "solve" a device without neutral access is dangerous — it can put an entire connected appliance's housing under voltage. This isn't specific to Shelly, it applies to all electrical wiring, but it's worth calling out because a missing neutral is exactly the situation that tempts people into this mistake (see the neutral section below).

Back box or DIN rail

This difference decides where in the house the device belongs. A Shelly Pro 4PM isn't meant to be squeezed in behind a light switch — it sits in the consumer unit, alongside the circuit breakers.

Devices that need a neutral — and the ones that don't

Most Shelly relays need both L and N to work, because the device's own electronics need both to get power, regardless of what it's switching.

Several Shelly products, on the other hand, are built for neutral-free operation — they draw their own power via the live conductor and suit older installations where only live and switch wires exist in the back box:

• Shelly 1L Gen3 and Shelly 2L Gen3 — the relay modules for one and two channels without a neutral (verified on kb.shelly.cloud).
• The older Shelly 1L — before Gen3 there was already an L model (the earlier L/NL variant) that ran without a neutral.
• Shelly's 230 V dimmers — all of them can run without a neutral.

At low load a bypass is often needed in parallel with the load, otherwise the module can't draw enough power and the lamp flickers or won't switch off fully. The threshold differs by generation:

On Shelly 2L Gen3 this applies only to the neutral-free channel O1; O2 always has a neutral and has no such restriction (up to 700 W).

There are three bypass models — pick the right one for your device:

• Shelly Bypass (legacy) — for Shelly 1L and Shelly Dimmer 2.
• Shelly Bypass for 1L/2L Gen3 — for Shelly 1L Gen3 and Shelly 2L Gen3.
• Shelly Bypass for Dimmer — for Shelly Dimmer Gen3 and Shelly Dimmer Gen4.

The bypass is wired in parallel with the load and draws a small residual current through it, so the Shelly module gets power without its own neutral. Maximum load is unaffected by the bypass. The same safety rules apply to the bypass module as to the relay itself: cut the power, verify it's dead, wire exactly to the diagram.

Common wiring mistakes

Based on what's verified about terminal markings (see Terminals and markings):

• Mistaking I for a control input. On devices like Shelly 1, I is the load circuit's input — where power to the lamp/motor enters — not a logic sensor input. SW is the control input for the switch.
• Assuming Shelly i4/Shelly Plus i4 have terminals marked I1–I4. They're marked SW1–SW4. The product name and the terminal names aren't the same thing.
• Assuming shutter wiring uses arrow-marked terminals. On two-relay devices like Shelly 2.5 and Shelly Plus 2PM, the terminals are still called O1/O2 in shutter mode — the ▲/▼ arrows are only verified on Shelly Pro Dual Cover/Shutter PM.
• Using the earth conductor as a stand-in for neutral. See the warning above — don't do this, no matter how tempting it is in a back box without a neutral.